Cons:
1) In retrospect, one huge expense of having a second child of a different gender was that in our market, we were basically forced to buy a house in order to get 3 bedrooms when our kids approached school-age. Then that started an ever-compounding roll of additional expenses related to repairs, commuting, utilities etc.
2) When you only have one child you are pretty well able to maintain some semblance of your previous identity as a single person and/or married couple with parenting just added on as a plus. When you have two kids at home, your central identity becomes "family." If you also have a career and a commute and a less than super-supportive partner, you may find yourself, as I did, having fantasies about forever locking yourself into a Woman's Restroom stall at some anonymous pull-off from I-80, simply because you had no responsibilities to fulfill while in there.
Pros:
1) Your child will have the great benefit of a sibling. MMV, but one of the reasons I chose to have a second child was that by age 2 I knew my DS30 was going to be an extreme introvert who might need somebody in his life who invites him to Thanksgiving when he is 64, and I am maybe dead. My DD28 is also an introvert, but much less extreme, so worked out just as I planned. Also, I was so madly infatuated with my infant son, I thought second child might help redirect or lessen my attention, and I was right about that too. As adults, I see each of my kids about once a month, and they hang out with each other at about that frequency too, and I think we are all happy with that. Which brings me to...
2) Having adult kids you can talk to over lunch who sometimes even pick up the bill

3) Parenting two different individuals will cause you to chill out on nature vs. nurture, because you will see so clearly that they were who they are from the moment they popped out into the light, but you will love both, or all of them, with the same rough, tender force.